Electric resistance element



k5 nyo 1 WM z 1 1 1 2, g 4 flwflo m 6 t C J N 6, n fi R 5 w m w m s r I.N T 2 H flag W a l A z 1 M 1 5 2111 Y 4. B F m m m m G. C. GAUT ET ALELECTRIC RESISTANCE ELEMENT Filed March 9, 1939 Sept. 23, 1941.

Patented Sept. 23, 1941 ELECTRIC RESISTANCE ELEMENT Geoffrey CharlesGaut, Romford,

Frederick Huggins, Forest Gate,

Beniamin London,

Donald Swaddle, llford, and Harold Vivien George Stubbs, Clacton-on-Sea,England, assignors to P. R. Mallory & 00., 1110., Indianapolis, Ind., acorporation of Delaware Application March 9, 1939, Serial No. 260,686 InGreat Britain April 6, 1938 Claims.

This invention relates to electric resistance elements.

An object of the invention is to improve such elements, particularlyelements made by photographic methods, such as are described in Gaut eta1. U. S. application S. N. 91,986, filed July 22, 1936 and in BritishPatent 460,895 corresponding thereto.

These elements, for example, may comprise sheets or strips of paper,Celluloid, cellulose acetate or the like coated with a layer of bindermaterial carrying dispersed therein finely divided conductive material,preferably produced photographically. The binder may, for example, be anitrocellulose (Pyroxylin) lacquer. The conductive material may comprisesilver developed in the layer by incorporating therein a silver saltsuch as silver bromide, chloride or iodide and exposing and developingphotographically. The silver deposit may be varied or graded across thelayer by controlling the time or rate of exposure prior to development.The layer may preferably also contain carbon in colloidal orsubstantially colloidal form (such as light soot) to increase itsconductivity. Both the silver and the carbon then contribute to itsconductivity.

Other objects of the invention will be apparent from the followingdescription and accompanying drawing taken in connection with theappended claims.

The invention comprises the features of construction, combination ofelements, arrangement of parts, and methods of manufacture and operationreferred to above or which will be brought out and exemplified in thedisclosure hereinafter set forth, including the illustrations in thedrawins.

In the drawing:

Figure 1 represents one method of coating the resistance elementsaccording to the invention.

Figure 2 is a section of a resistance element according to theinvention.

Figure 3 is a section through a modified element, and

Figure 4 is a perspective view of a complete resistance element.

While a preferred embodiment of the invention is described herein, it iscontemplated that considerable variation may be made in the method ofprocedure and the construction of parts without departing from thespirit of the invention. In the following description and in the claims,parts will be identified by specific names for convenience, but they areintended to be as generic in their application to similar parts as theart will permit.

According to the principal feature of the present invention an elementprepared in accordance with any of the said methods is improved by theaddition on one oneach side of a surface coating which is sumcientlywaterproof to reduce or prevent variations of resistance value due tomoisture. It is not essential that a completely waterproof coatingshould be provided but the coating should be sufficiently waterproof inthe sense that any resistance variations (due, for example to variationsin atmospheric humidity) that occur are within manufacturing tolerances.If the element proper, as is preferred, is carried on one side by a baseof paper or the like the water-proof surface coating need only beprovided on one side of the resistive layer. If the said layer is ofgraduated-resistance form so as to provide a variable-resistance elementthe surface coating should also be of such a character as to resist weardue to the co-operating movable contact. A coating which satisfies theseconditions and has no appreciable effect on the resistance of theelement may be obtained by using a lacquer formed from the followingingredients:

(1) Cellulose acetobutyrate grs s 8 (2) Carbon (20 mesh) grs 2 (3)Spermaceti wax grs 1.2 (4) Diamyl tartrate ccs 1.5 (5) Ethylenedichloride ccs In preparing the lacquer, ingredients 1 and 3 aredissolved in ingredients 4 and 5 and the carbon is then beaten in forhalf an hour. The resuiting lacquer, which is allowed to stand forfifteen minutes before use, is applied, in carrying out a continuousprocess as described in the prior patent, by means of a roller dippingin a bath containing a supply of lacquer and contacting with the exposedsurface of the carrier after development thereof; fixing is notnecessary, but we may pass the carrier after the developing stagethrough a bath of heptane or hexane before applying the surface coating.If fixing is carried out the coating is applied after fixing.

Instead of using the lacquer mentioned we may employ a carbon-containingsolution of any other suitable cellulose derivative (such as benzylcellulose) or a carbon-containing solution of polystyrol. The solventused should be one that does not soften or dissolve the resistanceelement.

As already stated, it is preferred that the resistance element propershould be carried by a base of paper or the like. If paper is used wehave found it desirable to protect the paper with the objects of,firstly, reducing absorption and transmission of water, and, secondly,preventing saturation of the paper with any of the liquids used in thedeveloping process. For this purpose we provide a protective coatingbetween the resistive layer and the paper (this reducing watertransmission through the paper to the said layer) and also on the otherside of the paper (this preventing saturation of the paper with any ofthe developing liquids). When these steps are taken and the free side ofthe resistive layer provided with the above-mentioned surface coatingthe resistance is rendered free from variations in value. Satisfactoryprotective coatings may be provided on the paper by applying twoseparate lacquers on each side thereof, for example, two lacquers formedby solutions of two different cellulose derivatives or one such solutionand a solution of polystyrol. The use of two separate lacquers (i. e.lacquers which do not fuse together after application) gives betterprotective qualities than can be obtained with a single lacquer. Thefirst coating applied to each side of the paper may be made inaccordance with either of the following tables:

Other cellulose derivatives, such as benzyl cellulose, may be used, or,as already stated, polystyrol. The lacquer made in accordance with TableA is slightly better than the other from the point of view of adhesionto the paper. If the first of these two lacquers is employed the secondcoating may be constituted as follows:

Cellulose acetobutyrate grs 150 Benzyl abietate grs i 30 Butyl stearaters Ethylene dichloride liters 4 whilst if the alternative is employed(Table B) the second lacquer may be made from the following ingredients:

Cellulose acetobutyrate grs 150 Benzyl abietate grs 30 Cellulose nitrate(low viscosity) grs 15 Butyl stearate grs 5 Ethyl acetate v 1ifer5 5 Wehave already mentioned that fixing is not necessary. Accordingly when ananti-fogging treatment is not employed the baths required for processingthe exposed element are reduced to three, including the bath of heptaneor hexane in addition to the flieveloping bath and a washing bathfollowing the developing bath. It is possible to reduce contamination ordeterioration of the developing bath by providing a seal of cyclohexaneon the surface of the developing liquid; the latter must have wateradded to it to cause the cyclohexane to float.

In order to provide improved contact with the ends of the element andgive low end values of resistance the exposed and developed layer may beprovided with a high-conductivity coating at each end, these coatingsbeing plated if desired; they should be provided before the element ispassed through the above mentioned bath of heptane or hexane. Thecoatings may be formed in any convenient way, e. g. with the use of asilver-containing solution, and the plating may be effected bytraversing the coating (acting as cathode) past a silver anode with theinterposition of a pad to which is supplied a' silver acetate or silvercyanide plating solution.

Referring to the drawing Figure 1 shows how the surface coating I2 isapplied to the strip ll] (of paper or the like) carrying the resistivelayer 1 I by passing the strip over a roller M dipping in a bath l3 ofone of the lacquers described herein.

Figure 2 illustrates a portion of the finished strip in cross section.Figure 3 is a cross section of the modified form in which the paperlayer I0 is coated on both sides with a protective lacquer layer I1,after which resistive layer H is applied and then the carbon-containingprotective layer I2 is applied over layer ll.

Figure 4 is a perspective view of a finished resistance elementcomprising a narrow strip of the paper II] carrying the resistive layerII on one side thereof and having deposited metal terminal layers I5 andH5 at the two ends of the resistance layer, the protective layer l2coating the resistance layer and terminals, although in some instancesit may obviously be left off the terminals or part of them. The elementmay be used flat or bent into a cylinder. Obviously a fiat circularelement could also be provided by punching from the original sheet.

Having now particularly described and ascertained the nature of our saidinvention and in what manner the same is to be performed, we declarethat what we claim is:

1. An electric resistance element for a moving contact variableresistance device comprising a.

base of sheet insulating material, a first layer of resistance materialthereon, said layer being subject to humidity changes, and a secondlayer of resistance material on top of said first layer, said secondlayer being resistant to moisture penetration, said first layercomprising finely divided silver suspended in a matrix of humiditysensitive material, the density of the silver particles in said matrixvarying over the length ofsaid layer. in a predetermined manner wherebya corresponding variation in specific resistance lengthwise of thelayeris obtained, said second layer providing a contacting surface forengaging the moving contact of said device.

2. An electric resistance element for a moving contact variableresistance device comprising a base of sheet insulating material, afirst layer of resistance material thereon, said layer being subject tohumidity changes, and a second layer of resistance material on top ofsaid first layer, said second layer being resistant to moisturepenetration, said first layer comprising finely divided silver suspendedin a matrix of humidity sensitive material, the density of the silverparticles in said matrix varying over the length of prising a lacquercontaining carbon suspended therein.

-3. An electric resistance element for moving contact variableresistance devices comprising a messes base oi sheet insulatingmaterial, a first layer of resistance material thereon, said layer beingsubject to humidity changes, and a second layer of resistance materialon top of said first layer, said second layer being resistant tomoisture penetration, said first layer comprising finely divided silversuspended in a matrix of humidity sensitive material, the density 01 thesilver particles in said matrix varying over the length of said layer ina predetermined manner whereby a corresponding variation in specificresistance lengthwise of the layer is obtained, said base comprising apaper sheet having a protective lacquer coating on each side thereof.

4. An electric resistance element for a moving contact variableresistance device comprising a base oi sheet insulating material, afirst layer of resistance material thereon, said layer being subiect tohumidity changes, and a second layer of resistance material on top 01'said first layer, said second layer being resistant to moisturepenetration, said first layer comprising iinely divided silver suspendedin a matrix 0! humidity sensitive material, the density 0! the silverparticles in saidmatrlxvaryingoverthelengthotsaidm 25 in a predeterminedmanner whereby a corresponding variation in specific resistancelengthwise oi the layer is obtained, said second layer providing acontacting surface for engaging the moving contact of said device andcomprising a lacquer containing carbon, a cellulose derivative and asolvent therefor.

5. An electric resistance element comprising a base of sheet insulatingmaterial, a first layer of resistance material thereon, said layer beingsubject to humidity changes, and a second layer of resistance materialon top of said first layer, said second layer being resistant tomoisture penetration, said second layer comprising a lacquer containingcarbon in proportions suiilcient to render said layer conductive in thedirection of its thickness, a cellulose compound of the type comprisinga water insoluble binder, and a wax compatible therewith sumcient torender said 20 layer resistant to the penetration of moisture.

GEOFFREY CHARLES GA'U'I. BENJAMIN FREDERICK HUGGINS. DONALD SWADDLI.

EAR/01D VIVIEN GEORGE STUBBB.

